Dressed all in black and wearing body armour he forced his way into the movie theatre through a fire escape and stood in front of the screen before throwing tear gas canisters into the audience.

The cold-hearted killer, armed with an assault rifle and two handguns, then began shooting at movie goers, who at first thought he was part of a publicity stunt for the film.

Hundreds of people, many in Batman fancy dress, were at the multiplex which was showing the film on at least three different screens such was the demand to see the third film in the Batman trilogy.

In a carefully planned attack he tossed several green gas canisters into the audience and afterwards positioned himself near an exit, where he knew people would flee through to try and escape the fumes.

After firing a shot into the ceiling he then fired wildly into the audience in front of him.

Horror: A man covered in blood is led out of the cinema (Photo: Youtube)

Wordlessly he then walked up the steps of the theatre shooting victims in their seats and as they cowered on the floor.

Some of the victims were in a neighbouring theatre room and were hit by bullets that passed through the walls.

Minutes later 24-year-old James Holmes was arrested, calmly surrendering without any struggle near a car outside the Century 16 multiplex theatre in the suburb of Aurora.

Officers then went to his nearby home which was booby trapped with explosives and evacuated the area.

Five hospitals treated the casualties, including a three-month-old baby and a six-year-old, for high-calibre gun shot wounds and gas inhalation.

Ten of the victims were pronounced dead at the scene and the two died at hospital. One of the deceased was a 12-year-old child.

Holmes's apartment in Aurora, five miles from the cinema, is now central to the police investigation and is rigged with explosives.

The 6ft 3ins tall enrolled at the University of Colorado School of Medicine a year ago but dropped out last month.

He was studying for a PhD in neuroscience but a university spokeswoman said she did not know why he withdrew.

In academic achievement, "he was at the top of the top", recalled Timothy P White, Chancellor at the University of California, Riverside, where Holmes earned his undergraduate degree before attending the Denver school.

Former student: James Holmes dropped out of med school last month (Photo: Handout)

It is also understood that he had no military background and had not been identified as a danger by any authorities, meaning he was on no watch list.

No indications that such an attack was being planned have been found on any social media websites.

Those who knew Holmes described him as a shy, intelligent person raised in California by parents who were active in their well-to-do suburban San Diego neighbourhood.

Holmes played soccer at Westview High School and ran cross-country before going to college.

The FBI said he had no "significant criminal history", with the only blip on his record a "minor" driving penalty from last year.

He attended high school in San Diego and reporters have flocked to the area where Holmes' family still live.

His family released a statement that read: “Our hearts go out to those who were involved in this tragedy and to the families and friends of those involved.

“We ask that the media respect our privacy during this difficult time.

"Our family is co-operating with authorities in both San Diego, California and Aurora, Colorado.

"We are still trying to process this information and we appreciate that people will respect our privacy.”

Video Loading

Video Unavailable

Click to playTap to play

The video will start in 8Cancel

Play now

Video will play in

Share this video

Watch Next

Speaking to ABC News, a woman identifying herself as Holmes' mother said that authorities "have the right person" and said she needed to "fly out to Colorado" as soon as possible.

Latest reports were that the gunman had at least three guns and may have had a shotgun as well as the rifle and two pistols.

The choice of film being shown at Century 16 may prove to be significant, as the gunman was wearing body armour and a mask over his face, similar to that worn by one of the characters in the new Batman film.

But as yet, there is no clear motive for the crime that has shocked a nation.

'He was very calm and just firing at random people': Witnesses describe the cinema gunman

Video Loading

Video Unavailable

Click to playTap to play

The video will start in 8Cancel

Play now

Video will play in

Share this video

Watch Next

Witness Trey Freeman was in the cinema as the 6ft 3in tall gunman burst in and threw a gas canister.

“It landed at the feet of a woman in front of me. The guy was very calm, it was so scary.

"He waited for it to explode before he began firing. The first shot was into the air and then started shooting at the audience.

“He was just letting off shots, he did not seem to be firing at anyone in particular. He didn’t say a word. It was terrible, he was shooting little kids and adults.

"We saw people with their faces covered with blood.”

Another witness described a “guy slowly making his way up the stairs and firing, picking random people”.